1. Field of the Invention
The present invention relates to a technique to examine durability of a membrane electrode assembly used for a polymer electrolyte fuel cell.
2. Description of Related Art
As a method for examining durability of a polymer electrolyte fuel cell, there has been known a technique described in Japanese Patent Application Publication No. 2006-120545 (JP 2006-120545 A), for example. JP 2006-120545 A describes a method for evaluating accelerated deterioration of an electrolyte membrane. In the evaluation method, an electrolyte membrane is provided in a container in which an aqueous electrolyte is poured, as a diaphragm that divides the aqueous electrolyte into two, and a positive electrode is provided in one of them and a negative electrode is provided in the other one of them. A given voltage is applied between the positive electrode and the negative electrode, and a current flowing between the positive electrode and the negative electrode is measured. Hereby, a deterioration of the electrolyte membrane is evaluated based on an increase in the current thus measured.
In the meantime, as a durability test method of a membrane electrode assembly (hereinafter also referred to as MEA) constituted by an electrolyte membrane, a catalyst electrode layer, and a gas diffusion layer, it is also possible to use such a test method in which a given voltage is applied to a membrane electrode assembly MEA like JP 2006-120545 A and its durability is examined. However, in consideration of such a case where the MEA as a test object is used as a product after the durability test, if a test period is long, the MEA may be damaged, which would be a problem. Further, in consideration of mass production of the membrane electrode assembly MEA, if a test period is long, productive efficiency decreases, which would be a problem.
Further, there has been known such a durability test method in which a voltage is applied to an MEA while the voltage is swept, and a current flowing in the MEA is examined. In this test method, a peak (hereinafter also referred to as a carbon oxidation peak) of a current caused along with oxidation of a catalyst carrying carbon included in a catalyst electrode layer appears in a measured current curve. In order to distinguish the carbon oxidation peak and a current flowing at the time when dielectric breakdown occurs in the MEA, a current value should be measured within a range including the carbon oxidation peak. On the other hand, the following fact is found by experiments that in a case where a sweep time of a voltage to be applied is shortened in consideration of a case where the MEA as a test object is used as a product after the durability test, a current value of the carbon oxidation peak increases. In a durability test with a shortened sweep time of a voltage, a current of the carbon oxidation peak at the time when the sweep time of the voltage to be applied is shortened is large, and a current flowing at the when dielectric breakdown occurs in the MEA is also large. In such a durability test, when a large current value is detected, it is necessary to determine whether a phenomenon due to the current is caused due to the dielectric breakdown of the MEA or caused due to the carbon oxidation peak. Accordingly, it is necessary to measure the current at least in the range including the carbon oxidation peak. However, in order to measure such a large current value due to the carbon oxidation peak, test facility is upsized and a cost is increased, which would be a problem.